This section provides background information related to the present disclosure which is not necessarily prior art.
A clutch arrangement may serve in a motor vehicle with all-wheel drive, for example, in dependence on a difference in speed between a permanently driven primary axle and a driven secondary axle, which can be switched in, to transmit the driving torque of a drive unit to the secondary axle. In other applications, a clutch arrangement may serve as a replacement of an axle differential for the transmission of a driving torque to a half-shaft of an axle, as a block for a longitudinal differential of an all-wheel driven vehicle, or as a block for an axle differential.
The clutch arrangement has an input element and an output element that may be rotated relative to one another (e.g., an input shaft and an output shaft). The clutch arrangement furthermore has a friction clutch by which the input element and the output element may be operably effectively coupled to one another (e.g., to drive the output element by means of the input element to make a rotary movement or to brake it). The friction clutch is effective in dependence on a hydraulic pressure that prevails in a pressure space of the clutch arrangement. In addition, the clutch arrangement has a hydraulic pump with a first pump part that is rotationally fixedly connected to the named input element and with a second pump part that is rotationally fixedly connected to the named output element. The pump is driven by a rotary movement of the input element and of the output element relative to one another. The suction side of the pump is connected to a low pressure space, (i.e., to a space in which a lower pressure prevails than in the aforesaid pressure space of the clutch arrangement). The pressure side of the pump is connected to the pressure space.
When a speed difference is present between the input element and the output element of the clutch arrangement, the pump is driven such that hydraulic fluid is conveyed into the pressure space of the clutch arrangement to actuate the friction clutch. A friction locking coupling is hereby effected between the input element and the output element of the clutch arrangement, which in turn contributes to a reduction in the speed difference between the input element and the output element and, thus, to a reduced pump power.
The pump may have an inner leakage in dependence on the specific design (i.e., hydraulic fluid can move from the pressure space through the pump into the low pressure space). The pump power is, thus, reduced in accordance with the leakage rate. Since the viscosity of the hydraulic fluid is typically temperature dependent, the leakage rate and thus the pump power are also dependent on the temperature of the hydraulic fluid. This effect results in an undesirably high temperature dependence of the clutch characteristic which characterizes the dependence of the torque transmitted by the clutch arrangement on the speed difference between the input element and the output element.
A clutch arrangement of the above type is known from U.S. Pat. No. 5,310,388 in which a control valve is arranged at a pump outlet and has a bimetallic strip that releases or closes the pump outlet in dependence on the temperature of the hydraulic fluid. The temperature dependence of the viscosity of the hydraulic fluid is thereby compensated. This kind of temperature compensation is, however, undesirably complex and/or costly.